The invention pertains to a method and a device for gathering printed sheets, particularly signatures and/or individual sheets.
During the gathering process in a book binding system, an individual signature is respectively withdrawn from several collections of identical signatures, wherein the various decollated signatures are delivered to a collecting conveyor in order to form stacks.
Known gathering machines feature feeder stations that are arranged in a row and respectively consist of a magazine for accommodating a signature stack, a withdrawal drum with a gripper mechanism for taking hold of and withdrawing the respective bottom signature that is tilted away from the signature stack, and an intermediate support for temporarily depositing the decollated signature. The collecting conveyor features a collecting channel that is oriented perpendicular to the signature withdrawal direction, wherein pushers of a transport chain moving in said collecting channel strip the decollated signatures off the individual intermediate supports and gather the signatures into a book block on the collecting channel. Such a gathering machine that operates with a change of direction is described in DE 14 86 744 B. This machine furthermore features reciprocating intermediate tables for pre-accelerating the decollated signatures for the transport by the pushers of the collecting conveyor.
A gathering machine with feed stations according to the longitudinal withdrawal principle is known from DE 196 16 047 A1, wherein this gathering machine comprises a conveyor belt that carries a signature stack in a magazine such that a region of the folded edge remains exposed and respectively advances the bottom signature in a correspondingly cyclic fashion, as well as a withdrawal conveyor system that serves for receiving and additionally transporting the advanced signatures and is composed of acceleration rollers. A clamping conveyor consisting of an upper and a lower conveyor belt is arranged downstream of the withdrawal conveyor system and inclined toward the collecting conveyor, wherein this clamping conveyor deposits the decollated signatures on the stack being formed with a synchronous transport movement upstream of a pusher of the collecting conveyor. The signatures decollated by the feeder stations are delivered to the collecting conveyor without a change of direction.
One disadvantage of known decollating devices can be seen in that the weight of the signature stack rests on the respective bottom signature such that corresponding frictional forces are generated between the bottom signature to be withdrawn and the following signature of the stack. This can result in markings on sensitive surfaces. One also has to accept the risk of dragging along the following signature. Stack lifters and blast air nozzles are used for minimizing these frictional forces, wherein an air cushion is produced between the bottom signature and the following signature with the aid of blast air. However, this is frequently only possible regionally because the introduced blast air escapes from the stack in accordance with the principle of least resistance.